An update to the clang tooling detects more issues with the code
including a memory leak from the g_string_new() allocation. Clean up
the code to avoid the allocation and use ARRAY_SIZE while we are at
it.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230630180423.558337-9-alex.bennee@linaro.org>